1. Field of the Invention
The present invention relates generally to a lens barrel and, more particularly, to a lens barrel constructed to progress and retreat a lens housing such as a lens cylinder of a camera through cam grooves.
2. Related Background Art
A variety of constructions have hitherto been adopted as configurations of a lens driving cam groove holed in, e.g., a lens cylinder. Various constructions have been proposed for the cam groove formed by so-called mold forming especially in the lens cylinder manufactured from a plastic material.
Generally, a lens driving oblique cam groove holed in the molded cylinder is forced to undergo many constraints in terms of structure of a molding die thereof. For this reason, the following construction has been proposed. This cam groove is formed as a bottomed cam groove in the inner peripheral surface of the cylinder. At the same time, a sectional configuration of the cam groove is designed to be a trapezoid, thereby facilitating a removal of the die. In the bottomed cam groove assuming the trapezoid, a backlash is easy to occur with respect to a cam follower. A biasing member for thrusting the cam follower against the cam groove is required for obviating the backlash.
Further, a cam groove assuming a shape other than the trapezoid in section by partitioning the molding die is also manufactured into a product. For manufacturing this type of cam groove, a parting line for die matching is needed in the cam groove. However, a pressure of the plastic material injected during mold forming is applied on this parting line. The plastic material permeates in the flash line between the mutual dies. So-called burrs are thereby produced.
Then, if the burrs are produced, the action of the cam follower fitted in the cam groove is hindered due to the burrs. Runs-offs 10B, as illustrated in FIG. 8, are formed for removing the burrs during the molding process. The parting line is partially retracted from a cam groove 10A so that the burrs are intruded inwardly of the wall surface of the cam groove 10A. The run-offs are thus formed.
In the lens barrel including the cam groove 10A described above, however, the backlash is caused in the run-offs 10B provided in the cam groove 10A. An optical performance of the lens barrel is thereby deteriorated. Besides, whereas if the run-offs 10B are not formed, as explained above, the action of the cam follower fitted in the cam groove 10A is hindered by the burrs protruding from the wall surface of the cam groove 10A. The operation becomes unstable.